Report errors when dependency resolution fails (#71)

* Report errors when dependency resolution fails

Function `Lib.reolveDependencies` now checks the result of the coursier dependency
resolution for errors. The return value of the fuction is now a
`Result[Seq[PathRef]]` and errors are signaled by returning an instance
of Failure when errors occur while resolving dependencies.

`ScalaModule` now has a new task called `resolve` which allows for
triggering dependency resolution directly from the cmd line like:
`mill run resolve`.

`ResolveDepsTests` was added to verify failed resolution is properly
detected and reported.

* Remove type signatures in ScalaModule.scala

* Updates from PR comments

4 files changed, 76 insertions, 17 deletions

diff --git a/scalaplugin/src/main/scala/mill/scalaplugin/Lib.scala b/scalaplugin/src/main/scala/mill/scalaplugin/Lib.scala
index 637906a5..c89254b0 100644
--- a/scalaplugin/src/main/scala/mill/scalaplugin/Lib.scala
+++ b/scalaplugin/src/main/scala/mill/scalaplugin/Lib.scala
@@ -6,7 +6,7 @@ import java.net.URLClassLoader
 import java.util.Optional
 
 import ammonite.ops._
-import coursier.{Cache, Fetch, MavenRepository, Repository, Resolution}
+import coursier.{Cache, Fetch, MavenRepository, Repository, Resolution, Module => CoursierModule}
 import mill.define.Worker
 import mill.eval.{PathRef, Result}
 import mill.util.Ctx
@@ -159,7 +159,7 @@ object Lib{
                           scalaVersion: String,
                           scalaBinaryVersion: String,
                           deps: Seq[Dep],
-                          sources: Boolean = false): Seq[PathRef] = {
+                          sources: Boolean = false): Result[Seq[PathRef]] = {
     val flattened = deps.map{
       case Dep.Java(dep) => dep
       case Dep.Scala(dep) =>
@@ -171,15 +171,30 @@ object Lib{
 
     val fetch = Fetch.from(repositories, Cache.fetch())
     val resolution = start.process.run(fetch).unsafePerformSync
-    val sourceOrJar =
-      if (sources) resolution.classifiersArtifacts(Seq("sources"))
-      else resolution.artifacts
-    val localArtifacts: Seq[File] = scalaz.concurrent.Task
-      .gatherUnordered(sourceOrJar.map(Cache.file(_).run))
-      .unsafePerformSync
-      .flatMap(_.toOption)
-
-    localArtifacts.map(p => PathRef(Path(p), quick = true))
+    val errs = resolution.metadataErrors
+    if(errs.nonEmpty) {
+      val header =
+        s"""|
+            |Resolution failed for ${errs.length} modules:
+            |--------------------------------------------
+            |""".stripMargin
+
+      val errLines = errs.map {
+        case ((module, vsn), errMsgs) => s"  ${module.trim}:$vsn \n\t" + errMsgs.mkString("\n\t")
+      }.mkString("\n")
+      val msg = header + errLines + "\n"
+      Result.Failure(msg)
+    } else {
+      val sourceOrJar =
+        if (sources) resolution.classifiersArtifacts(Seq("sources"))
+        else resolution.artifacts
+      val localArtifacts: Seq[File] = scalaz.concurrent.Task
+        .gatherUnordered(sourceOrJar.map(Cache.file(_).run))
+        .unsafePerformSync
+        .flatMap(_.toOption)
+
+      localArtifacts.map(p => PathRef(Path(p), quick = true))
+    }
   }
   def scalaCompilerIvyDeps(scalaVersion: String) = Seq(
     Dep.Java("org.scala-lang", "scala-compiler", scalaVersion),
@@ -195,4 +210,4 @@ object Lib{
     "#!/usr/bin/env sh",
     "exec java -jar \"$0\" \"$@\""
   )
-}
\ No newline at end of file
+}
diff --git a/scalaplugin/src/main/scala/mill/scalaplugin/ScalaModule.scala b/scalaplugin/src/main/scala/mill/scalaplugin/ScalaModule.scala
index 9f9425a4..ce7b5e00 100644
--- a/scalaplugin/src/main/scala/mill/scalaplugin/ScalaModule.scala
+++ b/scalaplugin/src/main/scala/mill/scalaplugin/ScalaModule.scala
@@ -2,13 +2,12 @@ package mill
 package scalaplugin
 
 import ammonite.ops._
-import coursier.{Cache, MavenRepository, Repository, Resolution}
-import mill.define.Task
+import coursier.{Cache, MavenRepository, Repository}
+import mill.define.{Source, Task}
 import mill.define.Task.{Module, TaskModule}
 import mill.eval.{PathRef, Result}
 import mill.modules.Jvm
 import mill.modules.Jvm.{createAssembly, createJar, interactiveSubprocess, subprocess}
-
 import Lib._
 trait TestScalaModule extends ScalaModule with TaskModule {
   override def defaultCommandName() = "test"
@@ -101,6 +100,7 @@ trait ScalaModule extends Module with TaskModule{ outer =>
       sources
     )
   }
+
   def externalCompileDepClasspath: T[Seq[PathRef]] = T{
     Task.traverse(projectDeps)(_.externalCompileDepClasspath)().flatten ++
     resolveDeps(
@@ -115,6 +115,7 @@ trait ScalaModule extends Module with TaskModule{ outer =>
       sources = true
     )()
   }
+
   /**
     * Things that need to be on the classpath in order for this code to compile;
     * might be less than the runtime classpath
@@ -140,8 +141,9 @@ trait ScalaModule extends Module with TaskModule{ outer =>
         Seq(dep)
       )
       classpath match {
-        case Seq(single) => PathRef(single.path, quick = true)
-        case Seq() => throw new Exception(dep + " resolution failed")
+        case Result.Success(Seq(single)) => PathRef(single.path, quick = true)
+        case Result.Success(Seq()) => throw new Exception(dep + " resolution failed")
+        case f: Result.Failure => throw new Exception(dep + s" resolution failed.\n + ${f.msg}")
         case _ => throw new Exception(dep + " resolution resulted in more than one file")
       }
     }
diff --git a/scalaplugin/src/test/resource/resolve-deps/src/main/scala/Main.scala b/scalaplugin/src/test/resource/resolve-deps/src/main/scala/Main.scala
new file mode 100644
index 00000000..5dcbe39a
--- /dev/null
+++ b/scalaplugin/src/test/resource/resolve-deps/src/main/scala/Main.scala
@@ -0,0 +1,3 @@
+object Main {
+  println("ResolveDeps Main: hello world!")
+}
diff --git a/scalaplugin/src/test/scala/mill/scalaplugin/ResolveDepsTests.scala b/scalaplugin/src/test/scala/mill/scalaplugin/ResolveDepsTests.scala
new file mode 100644
index 00000000..cd6bd40f
--- /dev/null
+++ b/scalaplugin/src/test/scala/mill/scalaplugin/ResolveDepsTests.scala
@@ -0,0 +1,39 @@
+package mill.scalaplugin
+
+import coursier.Cache
+import coursier.maven.MavenRepository
+import mill.eval.Result.{Failure, Success}
+import mill.eval.{PathRef, Result}
+import utest._
+
+object ResolveDepsTests extends TestSuite {
+  val repos = Seq(Cache.ivy2Local, MavenRepository("https://repo1.maven.org/maven2"))
+
+  def evalDeps(deps: Seq[Dep]): Result[Seq[PathRef]] = Lib.resolveDependencies(repos, "2.12.4", "2.12", deps)
+
+  val tests = Tests {
+    'resolveValidDeps - {
+      val deps = Seq(Dep("com.lihaoyi", "pprint", "0.5.3"))
+      val Success(paths) = evalDeps(deps)
+      assert(paths.nonEmpty)
+    }
+
+    'errOnInvalidOrgDeps - {
+      val deps = Seq(Dep("xxx.yyy.invalid", "pprint", "0.5.3"))
+      val Failure(errMsg) = evalDeps(deps)
+      assert(errMsg.contains("xxx.yyy.invalid"))
+    }
+
+    'errOnInvalidVersionDeps - {
+      val deps = Seq(Dep("com.lihaoyi", "pprint", "invalid.version.num"))
+      val Failure(errMsg) = evalDeps(deps)
+      assert(errMsg.contains("invalid.version.num"))
+    }
+
+    'errOnPartialSuccess - {
+      val deps = Seq(Dep("com.lihaoyi", "pprint", "0.5.3"), Dep("fake", "fake", "fake"))
+      val Failure(errMsg) = evalDeps(deps)
+      assert(errMsg.contains("fake"))
+    }
+  }
+}